This invention relates to a creep-inhibiting device for use in an automotive vehicle equipped with an automatic transmission, and more particularly to a device of this kind which can be rendered operative or inoperative without causing a shock to a frictionally engaging element of the clutch, and with required responsiveness depending on the stepping action of the accelerator pedal.
In an automotive vehicle equipped with an automatic transmission having a fluid coupling such as a torque converter, the phenomenon can occur that, due to dragging torque of the torque converter, the vehicle moves forward or creeps against the driver's will when the vehicle stands with the shift lever shifted to a drive range (forward) position, creating a load on the engine. Such load on the engine, if applied at engine idle, causes a drop in the rotational speed of the engine, even resulting in engine stall. To avoid this disadvantage, the driver has to step on the accelerator pedal by an amount corresponding to the drag torque so as to prevent a drop in the rotational speed of the engine at idle. However, this is not desirable from the viewpoint of fuel curtailment. Therefore, it is desired that when the engine is at idle, the transmission is automatically brought into a neutral state to interrupt power transmission between the engine and the driving wheels, so that the driver is not required to step on the accelerator pedal by a large amount, for economy of fuel consumption. It is known that a creep-inhibiting device manufactured for this purpose can also provide an advantage that vibrations of the vehicle body are reduced during idling operation of the engine, which is particularly effective when the device is applied to front wheel-driven vehicles.
Although creeping of the vehicle can be avoided by reducing the torque transmission capacity of a frictionally engaging element used for starting of the vehicle, i.e. a first-speed clutch, to substantially zero upon stoppage of the vehicle, the torque transmission capacity has to soon be recovered in response to the engine load, i.e. the stepping amount of the accelerator pedal, in order to start the vehicle. Particularly when the driver is required to move his vehicle in a narrow space, for instance, to park his vehicle in a space between vehicles parked in line with the street, it is desired to maintain the clutch in a slip-permitting state so that a desired small level of power transmission capacity may be obtained through the clutch, which is proportionate to a fine stepping amount of the accelerator pedal. On the other hand, when the vehicle is started with no other vehicle running ahead, it is required to shorten the slip-permitting period to a minimum possible value, so as to promptly establish complete engagement of the clutch. Otherwise the clutch becomes engaged only after the engine speed has risen to a high level, resulting in a discomfortable shock and early wear of the clutch.
In a vehicle equipped with a conventional creep-inhibiting device, if the accelerator pedal is stepped on and then immediately restored to the idle position, the creep-inhibiting device will be promptly operated so that the whole engine and transmission system is abruptly relieved of torque which has largely twisted mounting rubber members supporting same to suddenly release torsional energy from the mounting rubber members, etc., thus causing a discomfortable shock. To avoid this, the creep inhibiting function of the creep-inhibiting device has to be exhibited with a time delay by a short period of time until the engine speed is sufficiently decreased, even after the accelerator pedal has been released.
Further, as is known, to increase the internal pressure of the first-speed clutch in good response to the stepping-on of the accelerator pedal, desirably the pressure within the clutch should not be reduced to zero even while the creep-inhibiting device is operating, but should be controlled in advance to a value as close as possible to and at the same time smaller than a pressure value above which the clutch becomes engaged against the force of a return spring provided in the clutch, i.e. the engaging pressure Pe. However, if the pressure within the clutch is thus increased to such value close to the engaging pressure in advance, such increased pressure can act as a residual pressure at subsequent shifting of the shift lever to the neutral position or to the reverse position, thus degrading the responsiveness of the clutch to a gear shifting action by the driver, particularly in cold weather.